1. Field of the Invention
The present invention relates to an anisotropic conductive adhesive (ACA) film, and more particularly, a triple layered ACA film adapted for enhancing the adhesion strength of a typical single layer anisotropic Conductive Film (ACF) or for enhancing the adhesion strength of the ACA film in flip chip bonding.
2. Description of the Related Art
As well known in the art, the electronic packaging is a wide-ranging and various system fabricating technique including all processes related to from semiconductor device to final product. According to the sharply developing recent semiconductor technology, more than millions of cells are integrated while non-memory devices are developing with a tendency toward many I/O pin numbers, large die size, heat emission by a large quantity, high electric performance and so on. However, the electronic packaging technique for packaging the above devices does not catch up with the rapid development of the semiconductor art.
The electronic packaging is a very important technology in determining the performance, size, price, reliability and so on of the final electronic product, and more particularly, increasing its importance further in recent electronic products which pursue high electronic performance, ultra small size/high density, very high speed, permanent reliability and so on.
In conformance to the above trend, the flip chip bonding technology is one of technologies for electrically connecting a chip to a substrate and is recently obtaining the popularity.
However, the conventional flip chip bonding technique employs a complicated bonding process using solder including the steps of: coating solder flux on a substrate; aligning the substrate having surface electrodes with a chip having solder bumps; reflowing the solder bumps; removing remaining flux; and filling and hardening underfill. This disadvantageously makes the process complicated and final products expensive.
For the purpose of simplifying the complicated process, attention is recently paid to a wafer-based packaging technique by which a process is carried out by coating a wafer with a polymer material having functions of flux and underfill. Besides, researches for flip chip bonding techniques using a conductive adhesive are actively being carried out. This techniques have the following advantages: Price is low due to use of a general solder flip chip, an ultra microscopic electrode pitch can be obtained, the technique is environmental friendly since it does not use flux or lead component, and processes can be carried out at a low temperature.
The conductive adhesive is generally classified into an anisotropic conductive adhesive (ACA) and an isotropic conductive adhesive, and is basically composed of conductive particles such as Ni, Au/polymer and Ag and one selected from thermosetting resin, thermoplastic resin and blend-type insulating resin, i.e. mixed combination thereof.
FIG. 1A is a sectional view for illustrating a conventional ACA film. Referring to FIG. 1A, the ACA film 10 is based upon polymer resin where fine conductive particles 20 are dispersed to impart conductivity to the film 10. The ACA film 10 is attached at both sides with releasing films 30.
FIG. 1B is a sectional view for illustrating a flip chip bonding state by using the ACA film shown in FIG. 1A. First, one releasing film 30 is removed from one face of the ACA film 10, and the ACA film 10 is heat-pressed by its exposed face on a substrate 50. Then the other releasing film 30 is removed from the other face of the ACA film 10, an IC chip 40 having bumps 45 is aligned with electrodes 55 on a substrate 50. The IC chip 40 and the substrate 50 are heat-pressed together with the ACA film 10 so that the bumps 45 and the electrodes 55 are mechanically and electrically connected due to the transformation of the conductive particles of the ACF.
However, where the substrate 50 is a glass substrate such as FR-4 having a large thermal expansion coefficient and the IC chip 40 is a Si chip having a relatively small thermal expansion coefficient, the reliability of a flip chip package is disadvantageously degraded under thermal cycle because of thermal stress owing to the difference between the thermal expansion coefficients.
In order to solve this problem, an improved ACA film as shown in FIG. 1C has been proposed where non-conductive particles are contained by at least 30% by weight to reduce thermal expansion coefficient. Referring to FIG. 1C, more non-conductive particles 60 having a diameter smaller than the conductive particles 20 are contained in the ACA film 10 unlike FIG. 1A so as to reduce the thermal expansion coefficient of the ACA film 10.
The adhesion strength of the ACA film 10 is created due to the polymer resin. However, where the conductive and non-conductive particles 20 and 60 are contained by a large amount in the polymer resin like this, the polymer resin is reduced with the effective area to degenerate the interface adhesion and interface toughness thereby deteriorating the reliability of the flip chip package.